5.1 Climate Change

A promise in Paris, and the gap that followed

In 2015, representatives from 196 countries gathered in Paris to sign the most ambitious international climate agreement in history. The Paris Agreement committed the world to keeping global warming well below two degrees Celsius above pre-industrial levels, with efforts to limit it to 1.5 degrees. It was hailed as a turning point -- proof that humanity could unite to face its greatest collective challenge.

Yet less than a decade later, the world was on track for roughly 2.7 degrees of warming by the end of the century. Emissions kept rising, fossil fuel production expanded, and extreme weather -- wildfires, floods, droughts, hurricanes -- grew more frequent and devastating. The gap between what scientists said was necessary and what governments were willing to do had become the defining tension of our era. Climate change, importantly, is not merely an environmental issue. It is a political, economic, social and moral challenge, and understanding it requires not only scientific literacy but the ability to think critically about power, justice and the choices societies make about their future.

How the planet keeps its warmth

To understand the crisis, you first have to understand the greenhouse effect -- which, in its natural form, is a good thing. It is a natural process by which certain gases in the atmosphere trap heat from the sun, keeping Earth warm enough to sustain life. Without it, the planet's average temperature would be about minus 18 degrees Celsius rather than the roughly 15 degrees we enjoy. Here is how it works: solar radiation passes through the atmosphere and warms Earth's surface; the surface radiates heat back toward space as infrared radiation; and greenhouse gases -- carbon dioxide, methane, nitrous oxide and water vapour -- absorb some of that outgoing heat and re-emit it in all directions, trapping warmth in the lower atmosphere.

The problem is what humans have done to this natural balance. Burning fossil fuels -- coal, oil and natural gas -- along with deforestation and industrial agriculture has dramatically increased the concentration of greenhouse gases. Carbon dioxide levels have risen from about 280 parts per million before the Industrial Revolution to over 420 ppm today. This enhanced greenhouse effect is warming the planet at an unprecedented rate. The crucial distinction to hold onto is between the natural greenhouse effect, essential for life, and the enhanced greenhouse effect, caused by human activity -- it is the latter that is driving dangerous change.

Politics, tipping points and justice

The Paris Agreement of 2015 was a landmark for several reasons. It was the first agreement to include nearly every nation -- 196 parties. It set a clear temperature target. And it introduced the concept of Nationally Determined Contributions (NDCs), under which each country sets its own emission-reduction targets. But its difficulties are equally instructive, and a balanced view names them. The targets are voluntary: NDCs are not legally binding, and countries face no penalties for missing them, which has led to widespread underperformance. There is a genuine equity debate: developing nations argue that wealthy countries, historically responsible for most emissions, should bear the greatest burden, while many developing nations still need affordable energy for growth and poverty reduction -- a tension worth presenting fairly rather than taking sides. There have been political reversals, as when the United States withdrew from the agreement in 2017, rejoined in 2021, and faced further uncertainty. And there is the stubborn gap between pledges and action: even if all current NDCs were fully met, the world would still warm by roughly 2.5 to 2.7 degrees.

Science adds urgency through the idea of climate tipping points -- critical thresholds beyond which changes become self-reinforcing and potentially irreversible, even if emissions later fall. Examples include Arctic sea-ice loss, where melting ice exposes dark ocean that absorbs more heat and melts more ice; Amazon rainforest dieback, where drought and deforestation could turn rainforest into savanna; permafrost thaw releasing stored methane; and ice-sheet collapse in Greenland and West Antarctica that could raise sea levels by metres over centuries. Some tipping points may be triggered between 1.5 and 2 degrees of warming, and one can cascade into another. Beneath all of this lies a profound question of justice: the nations most responsible for emissions are often not the ones most vulnerable to the consequences. Understanding climate change means holding the science, the politics and the ethics together.

In short

Climate change is driven by the enhanced greenhouse effect: human activities, chiefly burning fossil fuels, have raised atmospheric CO2 from about 280 to over 420 ppm, trapping more heat and warming the planet at an unprecedented rate. The natural greenhouse effect is essential for life; the human-caused enhancement is the danger. The 2015 Paris Agreement united 196 nations behind a target of well below 2 degrees, but voluntary NDCs, an equity debate worth presenting fairly, political reversals and a gap between pledges and action have left the world on track for roughly 2.7 degrees. Climate tipping points like Arctic ice loss and permafrost thaw add urgency, and the deep injustice -- that those most responsible are often not the most vulnerable -- means climate change must be understood as a scientific, political and ethical challenge at once.